Platinum Group Element Mineralization in "Ballrooms" of the J-M Reef of the Stillwater Complex, Montana
Platinum Group Element Mineralization in "Ballrooms" of the J-M Reef of the Stillwater Complex, Montana

by Harper, Matthew P

Abstract (Summary)

The J-M Reef of the Stillwater Complex, Montana (a large layered mafic intrusion), is one of the highest grade platinum group element (PGE) deposits known in the world, producing primarily palladium and platinum in a 3.4:1 ratio. “Ballrooms”of the Stillwater Complex are anomalously wide areas within or stratigraphically below the J-M Reef that host platinum group element mineralization. Ballrooms have two typical morphologies (type 1 and type 2); the first is an abrupt thickening of the mineralization that extends below the Reef Package and the second is a gentle widening of the Reef Package and associated reef mineralization to a width of over 6 m. Ballrooms are highly variable in size. Minimum dimensions for ballroom designation are a thickness (perpendicular to strike) of 6 meters and a length of 5 meters (parallel to strike).

Mineralization contacts are irregular but sharp and are characterized by a dramatic decrease in sulfide content (from one to two percent in ballrooms to only trace amounts,

Whole rock major and trace element compositions of rocks from ballrooms exhibit a strong geochemical control by cumulus phases. There are no significant major or trace element differences in the rocks from the two ballroom types. Moreover, cumulate mineralogy in ballrooms shows no variation from cumulate mineralogy in the JM Reef. Magnesium, Fe, and Cr exhibit a strong correlation with one another and the other major elements but do not correlate with Cu, Ni, and S. This indicates that Cu, Ni, and S were controlled by processes other than those controlling the distribution of the major elements in cumulus phases.

Cl-rich hydrous phases in the ballrooms (apatite and phlogopite) are evidence for the presence of Cl-rich fluids that interacted with melt in the mineralized zone, inferred to coincide with the growth of cumulus silicate phases. Pegmatitic textures also evidence the presence of fluid. The concentrated fluids played the major role in the formation of these anomalously rich ore morphologies.

This fluid likely originated when intercumulate melt became fluid saturated during crystallization of the cumulate pile at the base of the magma chamber and migrated upward as Boudreau (1999) suggests. This fluid appears to have been concentrated in some areas to form locally enriched areas of PGE mineralization (ballrooms). Areas of extensive fluid-melt interaction could produce type 2 ballrooms, while type 1 ballrooms were formed where there was little or no melt present when the upwelling fluid became sulfide saturated.

The fluid generation and migration may have been caused by an eruption of flood lava from the crystallizing magma chamber. It is possible that even a small eruption from the chamber could generate a large enough pressure decease to induce fluid saturation in the melt remaining in the cumulate pile. This process may have repeated each time lava erupted from the evolving chamber and created multiple sulfide horizons in the Stillwater Complex.

Evidence of sulfide remobilization and low temperature secondary alteration is abundant in ballrooms. The secondary alteration phases include sericite, zoisite/clinozoisite, serpentine, magnetite, pyrite, talc, and chlorite. A regional metamorphic event at 1.7 Ga that changed the Pb isotopic composition of the sulfides is likely the cause of the alteration. This low temperature hydrothermal event locally remobilized sulfides, chalcophile elements, and PGEs in the J-M Reef and ballrooms and may have variably depleted or enriched parts of the mineralization. This remobilization of sulfides, chalcophile elements, and PGEs has had a significant influence on the local distribution (centimeters to a few meters) of PGE bearing sulfides.